1. Field of the Invention
The present invention relates to a method used to fabricate a reticle, which is also commonly referred to as a photomask. The reticle includes a patterned layer through which radiation passes during the transfer of the pattern from the reticle to a substrate via photolithographic techniques.
2. Description of the Background Art
A typical reticle fabrication process begins with the formation of a substrate which typically includes a silicon-containing base layer such as a quartz layer, with a layer of chrome applied over the quartz, and a layer of chrome oxide transitioning to chrome oxynitride which is formed over the chrome layer. A photoresist material is commonly applied over the chrome oxide/chrome oxynitride layer. The photoresist material is pattern imaged by irradiation, and the image in the photoresist is developed into a pattern. Then the patterned photoresist is used as a mask for transferring the pattern to the chrome layer. The pattern in the chrome layer permits radiation to pass through portions of the reticle when the reticle is used in the fabrication of a substrate, such as a semiconductor substrate, where the pattern is transferred via photolithography to the semiconductor substrate. The chrome oxide/chrome oxynitride layer of the reticle substrate functions as an anti-reflective coating (ARC) during patterning of the chrome layer. However, the anti-reflective properties of this layer are not as effective for present day photoresist imaging radiation as they were for imaging radiation which was used with earlier photoresists used in the art of reticle fabrication.
Reticles which are used in combination with a stepper of the kind used for semiconductor fabrication are generally 6 inch squares which are about 0.25 inches thick. Such reticles can be fabricated in most 8 inch or larger processing chambers of the kind which are used to fabricate semiconductor wafers. However, since the reticle is subsequently used in a manner where imaging radiation will come down through the top and out the bottom, there cannot be any significant scratches on either surface of the reticle through which the radiation will pass. As a result, the tool used for reticle fabrication requires specialized reticle substrate handling devices and contact surfaces. For example, a robot blade which moves the reticle substrate may hold it only by the edges or corners of the substrate and within a specific distance from the edge of the substrate. The pedestal upon which the reticle substrate sits is designed for minimal contact with the substrate, where a raised lip touches the edge of the reticle substrate or a few protrusions from the pedestal contact the reticle substrate.
Currently, during formation of the reticle substrate, the quartz base layer is polished on both major surfaces, followed by physical vapor deposition of a radiation-blocking layer such as a chrome layer over one of the major surfaces. Toward the end of the deposition of the chrome layer, oxygen is added to the deposition chamber so that a chrome oxide is formed; subsequently a small amount of nitrogen (referred to as a nitrogen bleed) is added to the deposition chamber as well, so that chrome oxide transitions to chrome oxynitride. As previously mentioned, the chrome oxide/chrome oxynitride layer functions to reduce reflectivity of the chrome surface during pattern imaging of a photoresist which is applied over the surface of the chrome oxide/chrome oxynitride layer. The amount of reflectivity depends on the imaging radiation.
One of the preferred direct write tools for imaging the photoresist is a continuous wave laser which writes at a wavelength of about 257 nm or 198 nm. This direct write tool is available under the trademark of ALTA™ from ETEC Systems, Inc., Hillsboro, Oreg. The reflectivity of the chrome oxide/chrome oxynitride layer is on the order of about 14% at 257 nm. This is much higher than desired and is an artifact from earlier techniques used to imaging the photoresist, where the imaging wavelength of the radiation was in the range of 405 nm and this worked in combination with the composition of the chrome oxide/chrome oxynitride layer to produce a reflectivity on the order of less than about 10%. To compensate for the present reflectivity problem during imaging of the photoresist with the radiation tools used today, an organic antireflective coating (ARC) may be applied over the surface of the chrome oxide/chrome oxynitride layer.
The chrome layer is typically patterned using a plasma dry etch technique where the plasma is generated from a source gas of chlorine and oxygen. This plasma etchant tends not to attack the quartz base of the substrate, which needs to remain transparent to radiation, so that the pattern in the chrome will be properly transferred during fabrication of a semiconductor wafer, for example but not by way of limitation. However, while the chlorine/oxygen plasma does not attack the quartz base of the reticle substrate, the oxygen present in the plasma does attack the photoresist which is being used to transfer the pattern to the chrome layer. This causes faceting of the photoresist, which is commonly referred to as “resist pull back”, where the change in the critical dimension written into the photoresist is reflected in a change in the critical dimension of a pattern etched into the chrome. This is sometimes referred to as “CD loss”. For example, based on a current test pattern where the nominal feature size pattern in the photoresist is about 720 nm, the feature size produced in the chrome may be 60 nm to 70 nm larger, principally due to resist pull back effects. If, for example, and not by way of limitation, the smallest space that can be written on a typical ARF (193 nm) photoresist using a 198 nm wavelength continuous wave laser is in the range of about 110 nm, then due to the resist pull back, the smallest chrome space which can be written may be in the range of about 170 nm to 180 nm. If, for example, and not by way of limitation, the smallest space that can be written on a typical ARF (193 nm) photoresist using an e-beam writing tool, available from Toshiba or Hitachi, for example, is about 90 nm, then due to the resist pull back, the smallest chrome space which can be written becomes about 150 nm to 160 nm. It is readily apparent that if this photoresist pull back problem can be eliminated, the smallest chrome feature which can be obtained is substantially improved.
The importance of eliminating the photoresist pull back problem is even more important when phase shifting reticles are considered. At present these reticles make up about 25% of reticles produced, but this percentage is increasing as feature dimension requirements go to smaller feature sizes. Phase shifting reticles are designed to neutralize diffraction components of the imaging radiation which affects the width of the space which can be written in the chrome. One of the preferred methods of phase shifting is accomplished using diffraction slits at particular locations in the chrome pattern. For a binary mask where the smallest space which can be written is 100 nm, for example, the phase shifting slit would preferably be in the range of 30 nm. However, since 30 nm cannot be written, the phase shift is limited to the threshold of what can be written. By eliminating the photoresist pull back (eliminating the CD bias which occurs because of the resist pull back), then the threshold for phase shifting can be lowered, and the feature resolution and integrity can be improved.
U.S. Pat. No. 6,171,764 to Ku et al., issued Jan. 9, 2001 describes the kinds of radiation reflection problems which may occur in photolithographic processes. The description relates to semiconductor manufacturing processes which make use of a dielectric anti-reflective (DARC) layer to reduce reflected radiation during photoresist imaging. In particular, the difference between the Ku et al. invention and other known methods is based on the ordering of specific layers in the substrate used in the photolithographic process. In the Ku et al. method, the DARC layer is applied over a substrate, followed by a hard mask layer, and then a photoresist. This is said to compare with other known methods where the DARC layer is used between the photoresist layer and the hard mask layer. (Col. 3, lines 35-46.)
U.S. Pat. No. 6,607,984 to Lee et al., issued Aug. 19, 2003 describes a method of semiconductor fabrication in which an inorganic anti-reflection coating is employed and subsequently removed by selective etching relative to an underlying inorganic dielectric layer. (Col. 1, lines 61-67, continuing at Col. 2 lines 1-6.)
European Patent Application No. 99204265.5 of Shao-Wen Hsia et al., published Jun. 21, 2000, describes a semiconductor interconnect structure employing an inorganic dielectric layer produced by plasma enhanced chemical vapor deposition (PECVD). In accordance with a preferred embodiment of the invention, a metal layer upon which photoresist patterns are developed comprises a sandwiched metal stack having a layer of conducting metal (aluminum, titanium, and the like) bounded by an upper thin-film ARC layer and a bottom thin-film barrier layer, where at least the top layer is composed of an inorganic dielectric substance. The use of an inorganic dielectric top ARC layer is said to facilitate the use of thinner photoresist layers while preserving the integrity of the photoresist pattern for deep sub-micron feature sizes. (Col. 1, lines 56-58, continuing at Col. 2, lines 1-8.)
All of the references described above pertain to the use of an ARC in the production of semiconductor devices. The production of semiconductor devices is typically carried out using exposure of a photoresist to blanket radiation through a reticle, to provide efficiency of production. The photoresist exposure time through a reticle is typically in the range of seconds to a few minutes. Applicants' invention pertains to a direct write of a pattern on a photoresist which is used to transfer a pattern to a reticle of the kind which is subsequently used in semiconductor production. This direct writing of a pattern on the photoresist takes hours, commonly between about 8 and about 20 hours. As a result of the time period required for patterning the photoresist which is used to fabricate the reticle (as well as possible differences in the photoresist material), chemical reactions may take place in the photoresist which affect the critical dimension of the patterned photoresist. Since the photoresists used for reticle fabrication are chemically amplified photoresists, and the time required for writing the pattern so long, the deflection of imaging radiation off the substrate underlying the photoresist becomes more critical than it is during fabrication of a semiconductor device, where photoresist patterning is carried out by blanket radiation through a reticle for a short time period.
There is currently a need for improvement in the functionality of the ARC used in reticle fabrication, so that a reduction in reflectivity is achieved for the radiation wavelengths currently used in the imaging of reticle fabrication photoresists. In addition, there is a need for a means of eliminating, or at least significantly reducing, the photoresist pull back during etching of the chrome layer (or other similar radiation blanking layer) to provide better control of the critical dimension of a patterned reticle.